It floatsNew tool levitates molten materials

It floats

New MSFC tool levitates molten materials

March 9, 1998: A new lab tool doesn't exactly defy gravity, but
it does hold it at bay so scientists can run materials experiments that
could be spoiled if the samples touched a container wall.

"It's what I consider to be the next generation in containerless
processing," said Dr. Mike Robinson of the Electrostatic Levitator
(ESL) recently installed at NASA's Marshall Space Flight Center.

The ESL uses static electricity to suspend an object inside a vacuum
chamber. While that happens, a laser heats the sample until it melts, so
scientists can record a wide range of physical properties without contact
with the container.

To make improved alloys and other compounds we must understand the physical
properties that govern ingredients behave. These include:

Surface tension, the same effect that lets small bugs walk on water

Viscosity, how "thick" a liquid is

Heat capacity, how slowly heat is absorbed or released

Undercooling and nucleation,
how far below freezing it will stay liquid

Determining these properties precisely is difficult, because anything
that handles or contains a molten sample will alter the results. It can
dampen vibrations or rapidly cool the sample. In some cases, the metal is
reactive enough to damage its container.

One answer to this problem is "hands off" processing using
static electricity to levitate a small sample. This is possible with the
ESL, developed by Loral Space Systems
of Palo Alto, California, and recently donated to NASA's Marshall Space
Flight Center in Huntsville, Alabama.

"They wanted to give it to a national laboratory where it would
be available to the scientific community," Robinson said of Loral's
generosity. "We're the only national laboratory doing containerless
processing."

The ESL will complement research with NASA/Marshall's
Drop Tube Facility and with
flight facilities such as the TEMPUS electromagnetic
levitation furnace flown last year on the MSL-1
Spacelab mission. ESL holds the samples in full view of the detectors
for several minutes at a time, but they are still under the effects of gravity.
Drop Tube experiments are truly weightless, but for just 4.3 seconds, and
the samples quickly fall past detectors. TEMPUS and other orbital facilities
allow weightless experiments for long periods, but must be scheduled years
in advance on Spacelab missions.

The ESL suspends liquid samples, including metals, without the sample
touching a container and without the scientists handling equipment that
might alter measurements. This makes ESL a premier tool for investigating
fundamental physical properties of advanced materials, including undercooling
and metallic glasses.

The heart of the ESL is the vacuum chamber (left) containing
a pair of electrostatic plates and four electrodes that position the sample
being processed (right). The sample's position is determined from the shadows
cast on detectors as two lasers shine at right angles through the vacuum
chamber onto the sample.

Rise...

The ESL uses the same effect that makes freshly dried socks push away
from each other, although the application is controlled and precise. Two
large, horizontal electrode plates electrically charge the sample and repel
it upward until it balances between the two plates. Two smaller pairs of
electrodes position the sample horizontally. A high-power deuterium arc
lamp shines on the sample to replace the electrical charge the sample loses
as it emits electrons while hot.

To keep the sample centered, a sophisticated three-dimensional digital
feedback system controls the electrode charges. Two lasers (operating at
different wavelengths or colors and at right angles to each other) shine
through the vacuum chamber to cast a shadow on position sensors on the outside.
The ESL computer, in turn, uses those positions to calculate subtle changes
in the electrode charges, recenter the sample, and to aim the heating laser.
The power of the electrostatic levitators is limited, so samples can be
no more than 3 mm (0.12 in) in diameter.

Most experiments require melting a sample so scientists can record viscosity,
surface tension, volume, and undercooling. A 50-watt laser, controlled from
0 to 100 percent and with a spot size from 10 mm down to 0.5 mm, heats samples,
or controls cooling by supplying slightly less energy than the sample radiates.
Experiments can also be run without laser heating.

The whole system is held in place by
a special optical bench mounted atop an air-bearing table to isolate the
system from vibration.

Right: ESL team members - Larry Savage, project scientist
Jan Rogers, and Michael Robinson of NASA, and Doug Huie of Mevatec - check
a test in progress. Robinson is peering through the long-distance microscope
which is includes a TV camera. The vacuum chamber is just in front of Rogers.
Positioning lasers are in the foreground, and the heating laser is mounted
under the table.

... and shine

To make "hands off" measurements, the ESL employs several sensors
looking in through viewports in the vacuum chamber wall. A CCD video camera
behind a long-range microscope provides a magnified view of the sample illuminated
by a conventional lamp until the sample is hot enough to glow.

Images can be recorded as standard and high-speed video, and digitized
for analyses. A pyrometer, viewing through an infrared filter, measures
the heat radiated by the sample. The images collected by the position sensors
can also be used in scientific analysis. Scientists using the ESL may also
supply their own instruments.

While the samples "float"
in the ESL, they are not in a microgravity environment and convective flow
effects can occur. The ESL is one of a suite of instruments for studying
the physical properties. Others which NASA employs include drop towers,
in which a sample is in free fall for 2-3 seconds and passes quickly through
an instrument's field of view, and a variety of space-based facilities in
which samples are under microgravity for extended periods.

Optical ports ring the ESL vacuum chamber to admit light
from the heating laser (the beam passes through the at left), positioning
lasers (one port is at center), and lamps (such as the deuterium arc lamp
at right), and to allow diagnostic instruments to view the sample.

A large number of studies - primarily in undercooling
research - will use the ESL. Measurements will include thermophysical
properties such as heat capacity, viscosity, surface tension, and thermal
conductivity. The ESL can also accommodate measurements of nucleation temperatures
and rates, and solidification velocities.